Crosslinked polyrotaxanes are produced by crosslinking polyrotaxanes in which a capping group is introduced at each end of a pseudopolyrotaxane. In the case that a pseudopolyrotaxane is formed from a polyethylene glycol (hereinafter, also referred to as a “PEG”) and a cyclodextrin that includes the PEG, for example, the resultant crosslinked polyrotaxane has a structure in which linear molecules of the PEG thread through cyclodextrin molecules in a skewered manner and the cyclodextrin molecules are movable along the linear molecules (has a pulley effect). The pulley effect allows the crosslinked polyrotaxane to uniformly distribute tensile force applied thereto. The crosslinked polyrotaxane is therefore not likely to have cracks or flaws, i.e., has excellent characteristics that conventional crosslinked polymers do not have.
The pseudopolyrotaxanes used for production of crosslinked polyrotaxanes are generally produced by mixing a PEG and a cyclodextrin in an aqueous medium. Accordingly, the resultant pseudopolyrotaxanes are obtained in the form of an aqueous dispersion. Efficient formation of a polyrotaxane by introduction of a capping group to each end of a pseudopolyrotaxane with a chemically stable bond can be achieved by a reaction between a PEG with a —COOH group at each end and a capping group reactive with the —COOH group, such as a —NH2 group or a —OH group.
This reaction of introducing a capping group to each end of a pseudopolyrotaxane, however, is deactivated by moisture in the system. Thus, the reaction requires the absence of water in the reaction system, or the water content controlled to an extremely slight amount, for efficient proceeding thereof. In other words, the aqueous dispersion of pseudopolyrotaxane requires sufficient elimination of water by drying the aqueous dispersion after solid-liquid separation by, for example, centrifugation or filtration, or drying without such separation.
Patent Literature 1 discloses that suspension of a precipitate of a PEG/α-cyclodextrin inclusion compound (pseudopolyrotaxane) in water and heating of the suspension to 70° C. or higher lead to a decrease in the inclusion ability and release of cyclodextrin molecules. Therefore, drying the aqueous dispersion of pseudopolyrotaxane at 70° C. or higher may cause a decrease in the inclusion ratio. The decrease in the inclusion ratio deteriorates the pulley effect of the crosslinked polyrotaxane, whereby the desired properties are not achieved. Accordingly, aqueous dispersions of pseudopolyrotaxane have been mainly freeze-dried or dried under decreased pressure at 70° C. or lower.
For example, Patent Literature 2 discloses a method in which an aqueous dispersion of pseudopolyrotaxane added in acetone, and the pseudopolyrotaxane is precipitated and then filtered, and the resultant product is vacuum dried at room temperature. However, the moisture in the pseudopolyrotaxane cannot be sufficiently eliminated by replacing the medium with acetone and filtering. Accordingly, drying at room temperature cannot completely eliminate the moisture, and residual moisture inhibits the reaction of introducing a capping group at each end of the pseudopolyrotaxane.